Since the septic tank is generating heat, you might be able to use the area for plants that might otherwise be too tender to grow in your area. The microclimate directly above the tank could be used for drought-tolerant, heat loving perennials and groundcovers like sage, heather, lavender, and thyme. I'd steer away from large shrubs with invasive root systems, and stick with plants that are well-behaved, to avoid future problems with your septic system.